The present invention relates to a high-frequency detecting element and a high-frequency heater using the detecting element.
In a heater using high-frequency waves (e.g., microwaves), an object is heated mainly because water in the object absorbs the high-frequency waves. The object, when it contains enough water, is properly heated, but the heating continues even after water in the object has completely evaporated. The methods discussed below are available to prevent the heating from continuing after completion of water evaporation so that the object is heated only to an appropriate temperature.
The method disclosed in Japanese Published Unexamined Application No. 62-123226 uses a humidity sensor and an alcohol sensor. The humidity sensor and alcohol sensor are disposed in an exhaust duct and are used to sense water vapor and gas produced from an object in a heating chamber and exhausted from the chamber in order to select a particular cooking method.
The method disclosed in Japanese Published Unexamined Application No. 5-172884 uses a microwave sensor and a temperature sensor. A bridge circuit, comprising the microwave sensor and the temperature sensor, is disposed in a heating chamber. The microwave sensor is composed of a thermistor element for sensing the temperature of an electromagnetic-wave absorber as it heats up upon absorbing microwaves, and the temperature sensor is composed of an electromagnetic-wave reflector and a thermistor element. The electromagnetic-wave absorber and the electromagnetic-wave reflector face a heated object and the thermistor elements detect changes in the temperature of the electromagnetic-wave absorber and the electromagnetic-wave reflector during microwave heating.
The method disclosed in Japanese Published Unexamined Application No. 60-170188 uses a thermistor. Water vapor and gas produced from a heated object in a heating chamber are detected with an absolute temperature sensor or a gas sensor to control an increase in heating chamber temperature due to a heater, using the thermistor to stop the heating operation.
However, all of these methods suffer problems. The method disclosed in Japanese Published Unexamined Application No. 62-123226 has a problem of temperature sensor performance deteriorating due to soiling. The method disclosed in Japanese Published Unexamined Application No. 5-172884 has a problem of the thermistor is slow in responding to an increase in the temperature. The method disclosed in Japanese Published Unexamined Application No. 60-170188 has a problem of a reduction in accuracy due to indirect measurement of the temperature of a heated object.